Source : Webster's Revised Unabridged Dictionary (1913)

Service cap \Serv"ice cap\ or hat \hat\ (Mil.)
   A cap or hat worn by officers or enlisted men when full-dress
   uniform, or dress uniform, is not worn.

   Note: In the United States army the service cap is round,
         about 31/2 inches high, flat-topped, with a visor. The
         service hat is of soft felt of khaki color, with broad
         brim and high crown, creased down the middle.

Source : WordNet®

service cap
     n : a cap with a flat circular top and a visor [syn: {kepi}, {peaked
         cap}, {yachting cap}]